1. Field of the Invention
The present invention relates to a serial type dot printer for printing letters with dot matrix configuration.
Heretofore, in a dot matrix printer that is used for the output of a word processor or the like, the 24-dot type which constitutes one letter with a dot matrix of 24 rows .times. 24 columns has been occupying the mainstream. In recent years, however, there has been an increasing demand for higher density in an attempt to elevating the quality of printing by this method such as the outputting of chinese characters close to the quality obtainable by the use of the types.
With this in mind, there is desired manufacture of a high density printer that has a denser construction than 24 .times. 24 for one letter, for the same size of the letter as before. However, due to the reasons that will be given below, realizing a high density has been difficult to achieve for a wire dot type serial printer or a dot type line printer.
Namely, the wire dot type serial printer has a construction in which printing wires of as many as 24 are concentrated in a very small space of the printing surface Because of this, the printing wire has to be bent by the use of a guide or the like, from the junction surface between the driving unit that is arranged, for example, in a circular form and the printing wire to the printing surface Further, in a very narrow space in the junction surface between the driving unit and the printing wire, the printing wires have to be attached to the driving unit by such a method as soldering so as not to make a contact with the neighboring wires. Accordingly, to increase the density of the printing dots without changing the size for one letter, the thickness of the printing wires has to be decreased. However, with such a reduced diameter, the printing wires of the existing type tend to be broken due, for example, to the impact at the time of printing reducing the reliability of the printing. Moreover, junction of a large number 24 printing wires and driving units in a very narrow space in the junction surface is technically difficult to achieve. As in the above, manufacture of a printing head which permits a high density printing has been difficult from the viewpoint of reliability, manufacturability, and so forth.
For this reason, a dot type line printer that can carry out printing without the printing wires was developed. In this printer, short printing pins that are attached to a printing hammer are arranged linearly with equal separation in the direction perpendicular to the direction of motion of the printing medium over the length equal to the width of the printing medium. Then, one dot line is printed on a printing medium by rocking a hammer base, that has hammers mounted on it for the length of a pitch to the adjacent printing pin, under the driving of each hammer with appropriately coordinated timings. After completion of printing for one dot line, the printing for the next dot line is carried out by shifting the printing medium for a distance which is equal to one dot pitch in the direction perpendicular to the dot line, and by repeating the driving of the hammer base in the same manner as in the above. Since short printing pins instead of wires may be employed for this type of printer, it is only necessary to reduce the diameter of the tip of the printing pins in order to achieve high density However, this type of printer requires a large number of hammers which necessitates in turn a power supply with large capacity In addition, a large special type of driving device is also needed to rock a hammer base that has hammers attached to it. These lead to disadvantages that the printer is high in cost and large in size.
In contrast, in Japanese Patent No. 60-139460 there is disclosed an improved serial type dot printer. In this printer, printing pins are arranged on a straight line that has an inclination of a fixed angle with respect to the scanning direction of the printing mechanism. Further, for driving the printing pins there are provided flat springs corresponding to the printing pins, disposed alternately in up and down fashion, without using wires and without requiring numerous hammers and a special rocking device, as are required in the line printer.
However, in a dot printer of this kind, printed letters are constructed by dot matrix so that there frequently occur cases in which neighboring hammers are driven at the same time. When the magnetic circuits for driving hammers that are attached to the neighboring hammers are excited simultaneously, there is generated a magnetic interference, giving rise to a problem of miscoordination in the timing of printing.